It's worth noting that Machine for Pigs was riding high on the success of The Dark Descent, from sharing the name of Amnesia to being given lots of promotion on youtube. And it was slightly cheaper at $20.

SOMA by comparison is a brand new IP, it may have been undercut a bit by A Machine For Pigs' lukewarm reception (even though it's been stated repeatedly it was a different developer, most consumers probably wouldn't see/get the difference), and hasn't been played up on Youtube that much by comparison. It's also a bit more expensive at $30

Clearly though releasing a console version was a good move as that buffeted the sales by a significant margin if the SteamSpy numbers of about 50K sold is accurate (taking in GOG and other version, then that would leave PS4 maybe around 20-30K sales)

I would also like to think that this game will have legs through good word of mouth. The reviews and such have certainly been overwhelmingly positive and most user reviews have been as well.

That said, I personally feel the value of the "Let's Play" Youtube personality audience is overstated. This audience to me seems more interested in watching games being played than actually buying and playing them themselves. And if you watch SOMA, I strongly believe that you won't get the same level of satisfaction than if you play/interact with it yourself.